Device for drying wet coatings applied to substrates of low electrical conductivity

ABSTRACT

The invention relates to a drying device for drying wet coatings applied to substrates of low electrical conductivity comprising a series of parallel spaced-apart first electrodes of rod-like form between which is located a row of upstanding electrode spikes defining a second electrode. HT voltage is applied between the first and second electrodes and dries the coating on the substrate as it passes close to the electrode array.

United States Patent 1 1 Spengler 22 Filed:

[ DEVICE FOR DRYING WET COATINGS APPLIED TO SUBSTRATES OF LOW ELECTRICAL CONDUCTIVITY [76] Inventor: Walter Spengler, Strehlgasse 23,

' Biel-Benken, Switzerland Apr. 17, 1974 211 Appl. No.: 461,789

[ 1 Jan. 7, 1975 3,470,621 10/1969 Bennett 34/1 3,619,538 11/1971 Kallenborn 219/1061 3,681,558 8/1972 Grassman 34/1 X 3,740,257 6/1973 Roschler 34/1 X Primary Examiner-Carroll B. Dority, Jr. Assistant ExaminerHar0ld Joyce Attorney, Agent, or Firm-Wenderoth, Lind & Ponack [5 7] ABSTRACT The invention relates to a drying device for drying wet coatings applied to substrates of low electrical conductivity comprising a series of parallel spaced-apart first electrodes of rod-like form between which is located a row of upstanding electrode spikes defining a second electrode. HT voltage is applied between the first and second electrodes and dries the coating on the substrate as it passes close to the electrode array.

3 7 Claims, 5 Drawing Figures l as 5,

Patented Jan. 7, 1975 2 Sheets-Sheet 2 This invention relates to a device for drying wet coatings applied to substrates of low electrical conductivity.

In many cases where coatings consisting of wet substances, or material available in solutions, are to be applied to substrates, the problem is encountered of rapidly drying these substances on the substrate, so as not to delay further processing or transport of the coated substrate. A special problem of this kind is encountered in colour printing when half-tone impressions in different colours are to be consecutively printed on a substrate. Here, the previously printed colours should always be dry before another colour isprinted if the individual colours are to be prevented from running together, especially in the marginal areas of the newly added half-tones. The method practised up to now consists either in providing, in the presses concerned, comparatively long interim paths in order to allow time for the drying process or in removing the printed sheets from the machine after each colour run and storing them until dry. Both thesefinethods suffer from the disadvantage that they result in a comparatively low production rate and, thus, relatively higher costs.

The present invention provides a device whereby the drying time for wet coatings applied to substrates of low electrical conductivity, can be reduced to such an extent that it becomes of the same order of magnitude as the time required for applying the coating. It is therefore an object of this invention to make possible the production of a product with at least one coating applied on a substrate of the type referred to, which leaves the coating device or printing machine all ready for dispatch.

According to the present invention a device for drying wet coatings applied to substrates of low electrical conductivity comprises a. a plurality of first electrodes in the form of elongated rod-like electrical conductors disposed in parallel spaced relationship,

b. an electrically insulating sheath surrounding each first electrode,

c. a plurality of second electrodes in the form of pointed spikes extending substantially perpendicular to the first electrodes and arranged in at least one row with the or each row extending between each adjacent pair of first electrodes, such that the pointed ends of the spikes in each row are virtually equidistant from the adjacent pair of first electrodes, and with the pointed end of the spikes situated virtually in the same plane as the pair of first electrodes,

d. HT generating means for maintaining a voltage difference between the first and second electrodes, and

e. retaining means which supports both the first as well as the second electrodes, said retaining means defining a surface electrically insulated from the first electrodes which surface confronts the first electrodes and above which the second electrodes project.

Each of the first electrodes is preferably surrounded by a tube of vitreous silica which envelops its electrode at least throughout its effective length.

The first electrodes may be connected either in parallel or in series to the HT generating means. The retaining means may be a base plate common to the first and second electrodes and consisting of an electrically insulating material, which base plate can be incorporated in the coating or printing machine which is to be provided with the drying device of the invention.

An embodiment of the invention will now be described, by way of example, with reference to the drawings, in which:

FIG. 1 shows, in diagrammatic representation, a printing machine comprising an assembly of printing units with, interposed therebetween a plurality of drying devices in accordance with the invention,

FIG. 2 is a plan of one embodiment of drying device in accordance with the invention,

FIG. 3 is a sectional view along the line III/III of FIG. 2, and

FIGS. 4 and 5 are diagrammatic representations explaining the mode of operation of drying devices in accordance with the invention.

In FIG. 1, the reference numerals l, 2, 3 and 4 denote four printing units following upon each other in the direction a sheet 5 travels through the machine. For the sake of simplicity, these units are merely indicated in each case by an ink fountain (ll, 21, 31 and 41), a form roller (12, 22, 32 and 42), a printing cylinder (13, 23, 33 and 43) and a back pressure cylinder (14, 24, 34 and 44). Each sheet 5 travels on a table 15, provided at least between the printing units l-4. Apertures 16-19 are provided in this table which correspond to the working area of each of four drying devices 7, 8, 9 and 10.

One of the drying devices 7-10 is shown in more detail in plan in FIG. 2 and in cross-section in FIG. 3. Each of the drying devices 7-10 comprises a number of parallel tubes 25-28 of vitreous silica, four in the present case, containing a wire or rod-like conductor 29 which forms an electrode. Depending upon the ease with which it can be introduced, this wire or rod-like conductor is either threaded through all tubes in one piece, or separate conductors are used in each, the wire or rod ends projecting from the ends of each tube being connected in series or in parallel. FIG. 2 shows a onepiece wire 29 threaded through the cores of the four tubes 26-28. The free end 30 at the left extremity of the tube 25 serves as a connection to a high-tension line (not shown). The ends of the tubes 26-28 are embedded in, or clamped by, electrically insulating retaining means 35, 36. In the embodiment of device shown in FIG. 2, the tubes are unsupported between the retaining devices 35, 36, the latter being in the form of blocks of electrically insulating material which surround the bare wire sections beyond the tube ends and prevent sparkover to adjacent metal parts. Alternatively, the insulating properties of the devices 35, 36, can be dispensed with by employing a continuous electrically insulating tube extending the total length of the wire and bending the tube through at the points corresponding to the ends of the runs indicated by the individual tubes in FIG. 2. This offers advantages from the point of view of electrical insulation.

In the embodiment shown in FIG. 2, the devices 35, 36 are integral with a base plate 37 of electrically insulating material in which metal bars 38, 39 and 40 are embedded. These metal bars are provided with corona point electrodes 45 in the shape of narrow-pointed spikes inserted along the longitudinal axis of each bar at equally spaced intervals. The bars 38-40 locate in grooves in the base plate 27. The pointed electrodes 45 project above the base plate 37, extending through upper parts of the grooves which are filled with an electrically insulating compound 46 up to the level of the upper surface of the plate 37. On the side of the metal bars 38-40 opposite the pointed electrodes 45, at least one connecting bolt 47 is provided. In order to enable all the bars to be kept at an identical electrical potential, the bars are interconnected by means of a strap 48 of good electrically conducting material which is tightened against the underside of the base plate 37 by means of nuts screwed to at least one of the connecting bolts 47. The connecting bolts 47 may also be used for mounting the drying equipment on a conductive or insulating support (not shown).

The drying power obtainable from each drying device can be varied depending on the number of tubes 25-28 extending across the direction in which each sheet travels through the machine. These tubes may be arranged either on a single or (as shown in FIG. 1) on several separate base plates. The drying equipment illustrated in FIGS. 2 and 3 which is equipped with four tubes, is therefore by no means limiting. The number of tubes or electrodes 29 depends upon the maximum drying power required at a drying point and their distribution among separate base plates 37 is undertaken with a view to operating requirements such as easy cleaning and disassembly, as well as power regulation by switching individual tubes.

The operation of the above-described drying device is based on the following phenomena (see FIG. 4): If a pair of long, virtually parallel electrodes 51, 52, each enveloped by insulating material 53, is connected to one terminal of a high-voltage source and a second electrode'in the shape of a slim metal spike with one pointed end 54, virtually on the same level as the electrodes 51, 52 and situated in the centre between these electrodes, is connected to the other terminal of this high-voltage. source, then corona discharges are set up between the electrodes 51, 52 and the point electrode 54. These discharges follow the paths indicated in FIG. 4. It is a characteristic feature of arrangements of the type referred to that slim point electrodes produce corona discharges similar to those indicated by the reference numerals 55, 56, which, in dependence on the operating voltage, extend in the axial direction of the corona point electrode for a certain distance, thereafter to veer off along a curved path towards the counter electrodes. This phenomenon can be observed with di rect as well as alternating voltages.

In FIG. 5, if an electrically insulating substrate 57, on which an electrically more or less conductive coating element 58 containing a suitable solvent is present, is taken past the surface of the above-mentioned insulating material 53 in a position closely above it, then heat is produced at points where the path of the discharge crosses the coating element, the amount of heat developed depending, inter alia, upon the conductivity of the coating element. The amount of heat produced is particularly large when the arrangement operates with a high voltage at high frequency. The potential differences used in practice are in the order of magnitude of kV, frequencies between 10 and 50 kHz being particularly suitable.

An array of four vitreous silica tubes each of approximately 5 mm outer diameter, each having a wall thickness of approximately 1.5 mm and arranged as shown in FIG. 2 with an overall width b of approximately 50 mm, is capable of drying ink layers on art paper of approximately g/m travelling at a velocity of approximately 10 cm/s, to the point of being wipe-resistant. The operating speed depends upon the thickness of the applied coating and the thickness of the substrate and may be improved by increasing frequency and the applied voltage. During the drying process, the wet ink lights up in the manner of a St. Elmos fire.

Since the power requirement rises with the wall thickness of the insulating tube, due to the higher operating voltage then required, the tendency is to use very thin walled tubes. To arrive at the optimum wall thickness it is necessary to take into consideration mechanical strength. Although it is not necessary for the tube (e.g., of vitreous silica) to remain unsupported from end to end of the drying device, it is not advisable to embed or support it over its entire length if a high degree of efficiency is to be attained. However, it may well be advisable to arrange bridge supports at suitable intervals along the lengths of the tubes in order to obviate an excessive fracture hazard.

The above-described equipment is suitable for the drying of any wet coating such as ink, glue etc. on a substrate of low electrical conductivity such as paper, card, or a textile material.

What is claimed is:

l. A device for drying wet coatings applied to substrates of low electric conductivity, comprising a. a plurality of first electrodes in the form of elongated rod-like electrical conductors disposed in parallel spaced relationship,

b. an electrically insulating sheath surrounding each first electrode,

c. a plurality of second electrodes in the form of pointed spikes extending substantially perpendicular to the first electrodes and arranged in at least one row with the or each .row extending between each adjacent pair of first electrodes, such that the pointed ends of the spikes in each row are virtually equidistant from the adjacent pair of first electrodes and with the pointed end of the spikes situated virtually in the same plane as the pair of first electrodes,

d. HT generating means for maintaining a voltage difference between the first and second electrodes, and

e. retaining means which supports both the first as well as the second electrodes, said retaining means defining means defining a surface electrically insulated from the first electrodes which surface confronts the first electrodes and above which the second electrodes project.

2. A device according to claim 1, in which each of the first electrodes is surrounded by a tube of vitreous silica which envelops the first electrode at least throughout its effective length.

3. A device according to claim 1, in which the first electrodes are connected in series to the HT generating means.

4. A deviceaccording to claim 1, in which the plurality of first electrodes is formed from bent-back runs of a one-piece electric conductor.

5. A device according to claim 4, in which the said one-piece electric conductor is enveloped by a vitreous silica tube throughout its entire length.

6. A device according to claim 1, in which the second electrodes are arranged at virtually uniform intervals in the or each row, the or each of the rows of second electrodes being supported by an electrically conductive bar.

7. A device according to claim 1, in which the retaining means comprises a base plate and end blocks of electrically insulating material. 

1. A device for drying wet coatings applied to substrates of low electric conductivity, comprising a. a plurality of first electrodes in the form of elongated rod-like electrical conductors disposed in parallel spaced relationship, b. an electrically insulating sheath surrounding each first electrode, c. a plurality of second electrodes in the form of pointed spikes extending substantially perpendicular to the first electrodes and arranged in at least one row with the or each row extending between each adjacent pair of first electrodes, such that the pointed ends of the spikes in each row are virtually equidistant from the adjacent pair of first electrodes and with the pointed end of the spikes situated virtually in the same plane as the pair of first electrodes, d. HT generating means for maintaining a voltage difference between the first and second electrodes, and e. retaining means which supports both the first as well as the second electrodes, said retaining means defining means defining a surface electrically insulated from the first electrodes which surface confronts the first electrodes and above which the second electrodes project.
 2. A device according to claim 1, in which each of the first electrodes is surrounded by a tube of vitreous silica which envelops the first electrode at least throughout its effective length.
 3. A device according to claim 1, in which the first electrodes are connected in series to the HT generating means.
 4. A device according to claim 1, in which the plurality of first electrodes is formed from bent-back runs of a one-piece electric conductor.
 5. A device according to claim 4, in which the said one-piece electric conductor is enveloped by a vitreous silica tube throughout its entire length.
 6. A device according to claim 1, in which the second electrodes are arranged at virtually uniform intervals in the or each row, the or each of the rows of second electrodes being supported by an electrically conductive bar.
 7. A device according to claim 1, in which the retaining means comprises a base plate and end blocks of electrically insulating material. 